1. Field of the Invention
The present invention relates generally to plating baths and methods for monitoring the constituents contained therein. More particularly, the present invention relates to a method for systematically applying both ac and dc voltammetric signals to a plating bath solution in order to obtain ac and dc spectra which accurately indicate constituent concentration levels. The method can be used to maintain desired major and trace constituent concentrations in order to ensure optimal plating bath performance.
2. Description of Related Art
A typical plating bath solution is comprised of a combination of several different electrochemical constituents. The specific constituents vary depending upon the type of plating bath, but in general can be broadly divided into what are commonly known as major constituents and trace, or minor, constituents. The major constituents are those electrochemical constituents which make up about 2 to 50 percent of the total bath weight or volume. Trace constituents, on the other hand, are present in smaller quantities, usually less than one percent of the total volume. For example, in an acid cadmium plating bath, cadmium ions and sulfuric acid are major constituents, and organic addition agents, degradation products and chemical contaminants are typical trace constituents.
The concentration levels of both major and trace constituents are important determinants of the quality of the resultant plating deposit. Trace constituent concentrations influence certain characteristics of the plating deposit, including tensile strength, ductility, solderability, uniformity, brightness and resistance to thermal shock. Monitoring and optimization of trace constituents assumes that the major constituent concentrations within the bath are already properly set and maintained. Should the major constituents fall outside of required concentration ranges, however, the bath may fail to satisfactorily perform its plating function. It is therefore important that both major and trace constituent concentrations be regularly monitored.
Current major constituent monitoring techniques typically involve removing a sample of the electrochemical solution from the plating tank for subsequent wet or instrumental chemical analysis. Typical methods of measuring major constituent content in various types of plating baths are disclosed in K. E. Langford and J. E. Parker, "Analysis of Electroplating and Related Solutions," pages 83-100, 65-68 and 174-180. Wet or instrumental chemical analysis methods such as these usually must be performed by highly skilled personnel. Specialized and costly chemical analysis equipment and supplies are required. Furthermore, the delay between drawing samples and receiving measurement results can be anywhere from several hours to several days. It is thus very tedious and expensive to monitor major constituent concentrations using currently available techniques. Moreover, the slow response time of some wet chemical analysis limits the extent to which a high quality plating bath can be continuously maintained.
Trace constituent monitoring techniques, however, can provide accurate results in real time without wet chemical analysis. The method disclosed in U.S. Pat. No. 4,631,116, assigned to the present assignee, uses ac voltammetry to produce response current spectra which vary as a result of changes in the concentration of various trace constituents. Other techniques, such as polarographic stripping, use dc voltammetric signals to analyze the plating bath. Both ac and dc voltammetric techniques have been found to produce accurate results in real time for trace constituent analysis. However, these techniques have not yet been considered for analyzing major constituents.
Furthermore, a single ac or dc voltammetric technique is unlikely to produce optimal measurement results for all trace constituents in all types of plating baths. In any constituent measurement, it is important that the measurement be selective, that is, substantially unaffected by other constituents in the bath. The measurement should also be very sensitive to the presence of the desired constituent. However, in certain cases, the ac or dc voltammetric signal response to one constituent is susceptible to interference from other constituents, which leads to decreased sensitivity and ambiguous measurement results. For example, the ac technique of U.S. Pat. No. 4,631,116 does not produce completely unambiguous response current spectra for multiple organic addition agents in the acid cadmium bath discussed above.
Under current practice, therefore, no single technique or set of equipment is sufficiently flexible to measure a wide variety of major and trace constituents. Wet or instrumental chemical analysis is typically used for major constituent measurement, and a variety of ac or dc techniques for trace constituent measurement. No integrated measurement system is available which combines the attributes of several measurement techniques. As a result, plating bath users must bear the additional expense of maintaining several sets of equipment for measuring different constituents. Furthermore, there is no efficient strategy for selecting among the various voltammetric techniques available. Users must rely on trial and error to select the most appropriate ac or dc technique for a given application.
As is apparent from the above, there presently is a need for an improved method of monitoring plating bath major and trace constituents which combines and extends the capabilities of existing techniques. The method should provide a systematic strategy for obtaining the most selective and sensitive response spectra for measuring a particular major or trace constituent. Furthermore, the method should be capable of providing the advantages of different voltammetric techniques using a single set of measurement equipment, resulting in a flexible and efficient overall plating bath analysis system.